The present invention relates to a sensor for the noninvasive measurement of liquids within a container, and in particular, to a sensor which can be used to indicate when liquid within the container drops below a desired level.
Medical science often requires that liquids be administered to a patient in a variety of situations. These liquids include simple intravenous feeding solutions, saline solutions for providing pressure to the eye during ocular surgery, contrast media infused to enhance imaging abilities, blood administered during transfusions, and nutrient solutions delivered via an enteral feeding pump. In virtually all such situations, it could be dangerous for the liquid supply to inadvertently "run dry." In some applications, allowing the container to run dry may decrease the pressure of the liquid below that desired. In other situations, it can result in air entering the blood stream, causing complications or even death.
Several approaches have been suggested for monitoring containers of liquids so that inadvertent "running dry" can be avoided. For example, one system involves use of an electrical needle skewered into the bottom of a bottle containing liquid to be monitored. A constant electrical current is applied to the needle, and when the liquid level drops below the end of the needle, the break in the electrical current causes a lamp to light on a master control panel. A major disadvantage of this approach is that the fixed location of the needle results in a fixed triggering position. Thus, the user cannot select the liquid level at which the system will signal that the container needs to be refilled or replaced. And yet, the desired triggering position of the indicator may vary for different medical procedures. Additionally, some procedures may benefit from being able to vary the location at which the indicator signal reacts during different periods of the procedure.
An additional disadvantage of this system is that it is invasive. By placing the needle in the solution, the risk of contamination is increased.
Yet another disadvantage of such a system is that it is limited in the types of fluids which may be monitored therewith. For example, this system is designed to work with an ionic solution, but will not work will many solutions which are not ionic.
Other available systems have ultrasonic liquid level detectors for blood containers in which the transducer is placed against an exterior wall of the container. Ultrasonic signals are emitted into the container and reflected signals are used to determine when the liquid level has dropped below a designated point. The coupling between the transducer and container sidewall, however, requires that gel be placed on the sidewall to conduct the ultrasonic signals from the transducer into the container and from the container back into the transducer. This approach can be time consuming and messy as gel must be applied to the transducer or sidewall each time the two are coupled.
Other systems, such as that disclosed in U.S. Pat. No. 5,303,585, teach fluid volume sensors to determine the volume of gas or liquid within a container. However, the sensors transmit the signals to remote processing units which indicate to the user whether the volume of liquid is below the desired level.
All of these systems lack provision of a simple, noninvasive, inexpensive and disposable sensor which may be easily installed on the monitored container without cumbersome cables or coupling gel. Thus, there is a need for a simple, inexpensive and disposable sensor which may be quickly and conveniently applied to a container to be monitored.
A feature common to all of the sensor systems mentioned above is that the sensors are designed for placement on jars, bottles and containers which have rigid sidewalls. This limitation is due to the type of sensor being used. Specifically, the sensors are designed to detect signal reflections from the container sidewalls. The sensors also require reliable contact when mounted flush to the container to assure transfer of the majority of signal energy between the sensor and the container at all times. Therefore, there is also a need for a system which will enable monitoring of the fluid level within the numerous types of containers which do not have rigid sidewalls, such as plastic bags. Such bags constantly change shape as fluids within the bags are drained, and which do not offer sidewalls that guarantee focused reflection of a signal in any particular direction.